European Patent Application EP 0 144 509 A3 "Fiber Optic interferometer transducer" teaches how a dual path polarimetric interferometer may be constructed so that, when suitable means convert isotropic pressure to anisotropic radial forces on an optical fibre, a useful signal results, which may be used as a measure of the isotropic pressure. Suitable means are described in the paper by K. Jansen and Ph. Dabkiewicz. "High pressure fiber-optic sensor with side-hole fiber", SPIE Proceedings, Fiber Optic Sensors II, Vol. 798 pp. 56-60 1987. where a single mode optical fibre is manufactured with side holes so as to form a polarimetric sensor sensitive to isotropic pressure. The dual path polarimetric interferometer operates by obtaining a measurement of the optical path length difference between the two orthogonal polarisation modes which are guided by the fibre. The optical path length difference is made intentionally sensitive to pressure, but is also generally sensitive to temperature which is often undesirable. Thus, such polarimetric pressure sensors normally require temperature compensation to be useful.
The output of the dual-path polarimetric interferometer sensors may be expressed in the form COS(.phi.), where the optical path length difference is expressed as a phase delay .phi. such that: EQU .phi.=(2.pi.L/.lambda.).(A T!+B T!.P)
and where T is the temperature. P the pressure. .lambda., the optical wavelength. L the length of fibre. A T! the inherent birefringence function of temperature, and B1 T! the pressure dependent birefringence function of temperature. When two lengths of fibre, designated 1 and 2. and of lengths L1 and L2. are joined together, and subject to pressures P1 and P2 respectively, then the combined phase delay is given by: EQU .phi.12=(2.pi./.lambda.).((L1.A1 T!+L2.A2 T!)+(L1.B1 T!.P1+L2.B2 T!.P2))
when the two orthogonal birefringent axes of the two fibres are aligned. and: EQU .phi.12=(2.pi./.lambda.).((L1.A1 T!-L2.A2 T!)+(L1.B1 T!.P1-L2.B2 T!.P2))
when the two orthogonal birefringent axes of the two fibres are at right angles.
In general, the temperature sensitivity of the inherent birefringence, A, is at least an order of magnitude greater than that of the pressure dependent birefringence, B, and so there is considerable advantage in miniraising the inherent component. If the two lengths are made identical, L1=L2=L. and the fibres rotated 90.degree. relative to each other about their longitudinal axis at the join, as taught by J.P. Dakin and C. Wade. "Compensated polarimetric sensor using polarisation-maintaining fibre in a differential configuration", Electron. Lett., Vol. 20, No.1, pp. 51-53, 1984. then A1 T!=A2 T!, and so the inherent birefringence component cancels to give: EQU .phi.12=(2.pi.L/.lambda.).(B1 T!.P1-B2 T!.P2)
If B1 T!=B2 T!, which will be the case if the two pieces of fibre are identically sensitive to pressure, then a useful signal will only be obtained if the two pieces of fibre experience different pressures, that is P1 is not equal to P2. This is the approach taught by the prior art, Dakin and Wade. However, there is an alternative approach in which a useful signal may be obtained when the two pieces of fibre experience the same pressure, that is P1 equals P2. If B1 T! may be made not equal to B2 T! and yet the expression A1 T!=A2 T! still applies, then P1 may equal P2, that is the two pieces of fibre may experience the same pressure, and yet temperature compensation of the inherent birefringence still occur. This is the basis for the present invention, where B1 T! and B2 T! are made to differ by changing the pressure experienced within side-holes formed in the fibre.
In some cases it may be advantageous to use unequal lengths of fibre in order to bias, the sensor at a convenient phase delay, especially when temperature compensation is only required over a limited range of operation. In a particular case, for example, the inherent birefringence component A1 T! and the pressure induced birefringence component B1 T!.P1 of one section may be compensated by the inherent birefringence A2 T! of the other section without compromising the overall pressure sensitivity. The present invention allows the adjustment of the lengths of fibre for this type of compensation. Also, it may be preferred to use unequal lengths of fibre so that the sensor has a particular temperature induced birefringence. For example, the overall temperature induced birefringence may be adjusted to be positive or negative with respect to the pressure induced birefringence by choosing the compensating fibre to be shorter or longer than the sensing section. In this case, the output of a number of sensors may be combined to differentiate the temperature and pressure induced birefringence changes. For example, it would be advantageous to combine two sensors, where one has positive and the other has negative temperature induced birefringence, but both have a similar value of pressure induced birefringence.
In order to achieve the separation of the two pressure regions as required by the prior art, some form of high pressure seal is required which normally acts upon the sensitive regions of the sensor. This leads to undesirable imperfections in the operation of the sensor which can be avoided with the present invention.
A different type of optical fibre sensor may be constructed by inserting partially reflecting mirrors into the fibre waveguide, such as by depositing a dielectric coating onto the end of the fibre and then splicing to another length of fibre, to form a Fabry-Perot interferometer as taught by T. Yoshini. K. Kurosawa. K. Itoh and T. Ose. "Fiber-Optic Fabry-Perot interferometer and its sensor applications", IEEE J. Quantum Electron., Vol. QE-18, pp. 1624-1632, 1982. Alternatively, partially reflected mirrors may be formed by writing a grating into the fibre as taught by G. Meltz. W. Morey, and W. H. Glenn. "Formation of Bragg Gratings in optical fibres by transverse holographic method". Opt. Lett., 14, pp. 823-825, 1989. Both these forms of sensor also require temperature compensation.